1. Field of the invention
The present invention concerns point-to-multipoint transmission networks linking a central station and a plurality of remote stations.
The present invention is more particularly concerned with networks of this kind which comprise between said central station and a concentration point a transmission medium which is time-shared between the various remote stations and between said concentration point and each remote station a respective transmission medium specific to each remote station and the lengths of which vary according to the location of the remote stations relative to the concentration point.
The present invention is applicable in particular to distribution networks used in the terminal part (close to the users) of a telecommunication network and particularly to distribution networks of this kind in which said transmission media are optical fibers and said concentration point is a passive optical coupler.
The present invention is more specifically concerned with the situation in which said time-sharing of the transmission medium utilizes the TDMA (Time Division Multiple Access) technique with a transmission format structured in frames comprising different time slots assigned in a predetermined manner to respective different remote stations.
For transmission in an "upward" direction from the remote stations to the central station the problem arises of identifying the time for transmission by each of the various remote stations so that when the information transmitted reaches the shared transmission medium it is in the respective time slots assigned to the various remote stations.
2. Description of the prior art
A known way to solve this problem is to implement at each remote station a specific transmission time-delay in order to compensate for the different transmission times to the central station from the remote station in question and a reference remote station.
The transmission time between a remote station and the central station is usually determined by measuring at the central station the time which elapses between a reference time at which information signals are sent by said central station and the time at which said central station receives a location signal transmitted by said remote station with a time-delay relative to a reference time at which said remote station receives said information signals determined in such a way that the various location signals sent by the various remote stations are received in a given period known as the location window which is not assigned to receiving information signals.
This transmission time measurement technique is described in French patent application N.degree. 2 636 482, for example.
It has the drawback that it requires a relatively wide location window covering all location. signal intermediate positions between respective extreme positions representing the remote station nearest the central station and the remote station farthest from the central station.